Human Embryonic Development

• trimesters (1st, 2nd and 3rd) - three-month periods of the 9 months from conception to birth
• Period of the egg (pre-embryonic period) - from fertilisation of the egg to the end of the 3rd week of development with implantation of the conceptus.
• Period of the embryo (period of organogenesis) - from the beginning of the 4th week to the end of the 8th week of development. This is the period when each of the three germ layers are formed and they give rise to specific tissues and organs. By the end of the Embryonic period, the main organ systems have been established.
• Period of the fetus - from the beginning of the 3rd month to birth. The period for the maturation of the embryonic organ systems and tissues.
• Gametogenesis - process of production of male and female gametes or sex cells (sperm and oocyte) from the primordial germ cells via meiotic cell division in the gonads (testes and ovaries).
• Primordial germ cells are specialised stem cells that give rise to the germ line and are formed a generation earlier when the parents were embryos.
• Gametogenesis in the male is called spermatogenesis and in the female oogenesis
• Gametogenesis reduces the chromosomal number of the gametes from a diploid (23 pairs - 46) to a haploid (23 unpaired)
• Gametogenesis results to an enhanced genetic variability in the gametes through a random recombination of genetic materials on homologous maternal and paternal chromosomes.
• Spermatogenesis does not start in the male until puberty, and occurs in the seminiferous tubules of the testes.
• Oogenesis starts from fetal life in the female, occurs in the ovary and completed at puberty.
• Stem cell - undifferentiated cell that has the ability to form specialised cell types. This could either be an embryonic or adult stem cell. Embryonic stem cells are pluripotent while adult stem cells are multipotent.
• Pluripotent cell - has the ability to form all mature cell types in the body except placental and extraembryonic cells. It cannot form a whole organism
• Multipotent cell - has the ability to form more than one closely related mature cell types in the body but not as varied as pluripotent cells e.g. cord blood/bone marrow stem cells form erythrocyte, leukocyte, platelet.
• Totipotent cell - has ability to form all differentiated cell types in the body including the placental and extraembryonic membrane cells. It could form a whole organism e.g. a zygote and first few generations of blastomeres.
• Errors in gametogenesis could lead to chromosomal abnormalities that could result in birth defect or spontaneous abortions.
• Errors in spermatogenesis could lead to a number of spermatozoa morphological abnormalities that could affect male fertility.
• Fertilisation is the process by which male and female gametes fuse to form a zygote and occurs at the ampullary region of the uterine tube (oviduct).
• The phases of fertilisation:

  • phase 1 - penetration of the corona radiata
  • phase 2 - penetration of the zona pellucida
  • phase 3 - fusion of the oocyte and sperm cell membranes
• Capacitation - a sperm conditioning process within the female reproductive tract (uterine tube) in preparation for fertilisation of the ovum. Involves epithelial interactions between the sperm and mucosal surface of the uterine tube. Only capacitated sperm can pass through the corona cells and undergo the acrosome reaction
• Acrosome reaction - this is induced by the zona proteins following binding of the acrosomal region of the sperm with the zona pellucida of the ooocyte. Acrosome reaction leads to the release of enzymes needed to penetrate the zona pellucida
• Cortical and zona reactions - Following the release of acrosome

  enzymes (acrosin), the sperm is able

  to penetrate the zona pellucida. The

  sperm’s contact with the plasma

  membrane of the oocyte leads to the

  release of lysosomal enzymes from

  cortical granules in the plasma

  membrane, which becomes

  impenetrable to other spermatozoa

  (cortical reaction); and to changes in

  the permeability of the zona pellucida.

  The enzymes alter the properties

  including structure and composition

  of the zona pellucida (Zona reaction)

  to prevent polyspermy.
• Fertilisation restores the diploid number of chromosomes as contained in typical body cells.
• The zygote that is produced after fertilisation has a unique genome different from the cells of the parents
• Fertilisation activates the egg to commence subsequent embryological development
• Physiological processes and anatomical framework relating to the release of the gametes and fertilisation are used as basis for most contraceptive methods.
• Male infertility could result from the quality and quantity of spermatozoa ejaculated
• Female infertility could result from a number of causes including physiological and anatomical factors.
• Infertility in both male and female could be treated with various forms of assisted reproductive technology.
• Cleavage starts within the first week following fertilisation. It is the repeated series of rapid mitotic divisions of the large zygote to produce an increasing number off smaller daughter cells called blastomeres
• Cleavage does not result in cell growth as there is no increase in the protoplasmic mass
• Cleavage increases the nucleocytoplasmic ratio, with each cleavage the cytoplasm is partitioned as nuclei are replicated leading to increased number of smaller cells which approach the size of a typical body cell.
• Cleavage transforms the zygote into a solid ball of cells called morula after about the 12-16 cells stage within about 3 days following fertilisation.
• On of the main purpose of the cleavage is to generate a multicellular embryo (morula) from a single large cell (zygote).
• Morula - a solid ball (mulberry) of 12-16 cells (blastomeres) following cleavage
• Compaction - a process of cells (blastomeres) reorganisation and segregation into inner cell mass (embryoblast) and outer cell mass (trophoblast) following cleavage. This involves the establishment of inside-outside polarity and increased maximised cell-to-cell contact
• Blastocyst - this describes a stage when the morula develops a fluid-filled cavity (blastocoel) with a compact inner cell mass at one side of the cavity (embryonic pole) enclosed by a thin, single-layered epithelium of trophoblast.
• Blastocyst formation progresses into the 2nd week following fertilisation
• Parthenogenesis - this is a process where an unfertilised egg goes on to develop into a new individual
• A cluster of embryonic cells (inner mass also called embryoblast) at the embryonic pole of the blastocyst that gives rise to tissues of the embryo proper, and constitutes the germ disc, formed within the 2nd week.